As mt_rand() can be seeded for repeatable sequences the current implementation makes it incompatible with other systems that do use correct implementations. However fixing it also means that the sequence generated for a given seed in PHP will also now be different.

The legacy implementation will be preserved and be selectable with a new `mt_srand(int $seed [, int $mode])` parameter, along with new constants representing the two modes. The default will be the fixed algorithm.